The present invention relates to a polishing apparatus for polishing a workpiece such as a semiconductor wafer to a planar finish, especially a device pattern on the surface of the semiconductor wafer by bring the surface of the semiconductor wafer in contact with a polishing cloth, and particularly to a method of conditioning the surface of a polishing cloth attached to a turntable in the polishing apparatus.
Recent rapid progress in semiconductor device integration demands smaller and smaller wiring patterns or interconnections and also narrower spaces between interconnections which connect active areas. One of the processes available for forming such interconnection is photolithography. Though the photolithographic process can form interconnections that are at most 0.5 xcexcm wide, it requires that surfaces on which pattern images are to be focused by a stepper be as flat as possible because the depth of focus of the optical system is relatively small.
It is therefore necessary to make the surfaces of semiconductor wafers flat for photolithography. One customary way of flattening the surfaces of semiconductor wafers is to polish them with a polishing apparatus, and such a process is called Chemical Mechanical Polishing (CMP) in which the semiconductor wafers are chemically and mechanically polished while supplying an abrasive liquid comprising abrasive grains and chemical solution such as alkaline solution.
In the polishing apparatus for polishing the surface of a semiconductor wafer, especially a device pattern on the upper surface of a semiconductor wafer, to a planar finish, as a polishing cloth attached to a turntable, a nonwoven fabric polishing cloth has heretofore been employed.
Higher levels of integration achieved in recent years for ICs and LSI circuits demand smaller steps or surface irregularities on the polished surface of the semiconductor wafer. In order to meet such a demand, it has been proposed to employ a polishing cloth made of a hard material such as polyurethane foam.
After the semiconductor wafers are contacted with the polishing cloth and polished by rotating the turntable and the top ring which holds the semiconductor wafer, the polishing capability of the polishing cloth is gradually deteriorated due to a deposit of abrasive grains and ground-off particles of the semiconductor material, and due to changes in the characteristics of the polishing cloth. Therefore, if the same polishing cloth is used to repeatedly polish semiconductor wafers, the polishing rate of the polishing apparatus is lowered, and the polished semiconductor wafers tend to suffer polishing irregularities. Therefore, it has been customary to condition the polishing cloth according to a process called xe2x80x9cdressingxe2x80x9d for recovering the surface of the polishing cloth before, or after, or during polishing.
There are basically two types of dressing processes, one of which is a contact-type dressing process in which a brush or a diamond dresser is brought into contact with a polishing cloth and rubs the polishing cloth, and the other of which is a noncontact-type dressing process in which a fluid jet of water or gas is applied under high pressure to the surface of the polishing cloth.
In the conventional polishing apparatus, either a dressing unit comprising a brush or a diamond dresser or a dressing unit employing a fluid jet has been incorporated therein, depending on the properties of a polishing cloth.
It has been found that when a new polishing cloth starts to be used, it needs to be dressed by a brush or a diamond dresser for initial conditioning, and while the polishing cloth is being used in a polishing process, it needs to be dressed by a fluid jet to remove an aggregate of abrasive slurry or ground-off particles of the semiconductor material therefrom. Unless the polishing cloth is dressed by the fluid jet, the polished surface of the semiconductor wafer is liable to be scratched, resulting in a poor yield of properly polished semiconductor wafers. For the above reasons, these two dressing units are required to be replaced with each other, when necessary, in the conventional polishing apparatus. Such a selective installing and replacing work has been tedious and time-consuming, and is liable to lower the throughput of the semiconductor wafers.
It is therefore an object of the present invention to provide a polishing apparatus which incorporates both a contact-type dressing unit having a brush or a diamond dresser and a noncontact-type dressing unit employing a fluid jet.
Another object of the present invention is to provide a method of conditioning the surface of a polishing cloth attached to a turntable in such a polishing apparatus.
According to one aspect of the present invention, there is provided a polishing apparatus comprising: a turntable with a polishing cloth attached thereto; a top ring for holding and pressing a workpiece to be polished against the polishing cloth under a certain pressure; a first dressing unit having a contact-type dresser for dressing the polishing cloth by bringing the contact-type dresser in contact with the polishing cloth; and a second dressing unit having a noncontact-type dresser for dressing the polishing cloth with a fluid jet applied thereto.
According to another aspect of the present invention, there is provided a method of conditioning a polishing cloth attached to a turntable for polishing a workpiece, comprising: dressing the polishing cloth with a first dressing unit having a contact-type dresser as initial conditioning when the polishing cloth starts to be used; and dressing the polishing cloth with a second dressing unit having a noncontact-type dresser between polishing processes, each for polishing the workpiece.
According to still another aspect of the present invention, there is provided a method of conditioning a polishing cloth attached to a turntable for polishing a workpiece, comprising: dressing the polishing cloth with a first dressing unit having a contact-type dresser as initial conditioning when the polishing cloth starts to be used; and dressing the polishing cloth first with the first dressing unit and then with a second dressing unit having a noncontact-type dresser between polishing processes, each for polishing the workpiece.
The polishing apparatus is equipped with both the contact-type dressing unit such as a diamond dresser and the noncontact-type dressing unit such as a fluid jet dresser. Therefore, it is not necessary to replace these contact-type and noncontact-type dressing units with each other on the polishing apparatus, but the contact-type and noncontact-type dressing units may be combined to carry out a desired pattern of dressing processes to dress or condition the polishing cloth.